Physics of Atomic Nuclei最新文献

Recently, the Large High Altitude Air Shower Observatory (LHAASO) published measurements of the all-particle CR energy spectrum and the mean logarithmic mass of CRs with unprecedented accuracy at 0.3–30 PeV. The mean logarithmic mass shows a nonmonotonic change with energy, a feature observed for the first time. In this work, we present a new approach to describe the mechanisms of formation of this feature. The key elements of this approach are the non-classical diffusion model of cosmic rays developed by the authors in which the knee in the observed spectrum occurs naturally without the use of additional assumptions, as well as power-law asymptotics before and after the knee, and a soft spectrum of particle generation in cosmic ray source. To obtain a more complete picture of the spectrum formation in the region of the knee and the sources that form it, we carried out calculations of the spectra of the main groups of nuclei in the energy range of 1 TeV–100 PeV. It is shown that the behavior of the all-particle spectrum and mass composition in the knee region are determined by local pevatrons located at a distance of 750–900 pc from the Earth. The energy of the knee practically coincides with the knees in the spectra of protons and helium nuclei. The contribution of the light components (p+textrm{He}) is about 70(%), the CNO group provides ({sim}13%). The energy spectrum index of the light components is ({-}2.61) before the knee. The nonmonotonic change in the mean logarithmic mass is due mainly to a decrease in the contribution of the CNO group in the energy range of 0.3–3 PeV.

Rare decays of the (D) mesons into vector light mesons are studied in detail within the framework of the relativistic quark model based on the quasipotential approach and quantum chromodynamics. The hadronic matrix elements of the weak current between meson states are calculated with the consistent account of relativistic effects. The invariant form factors that parameterize these matrix elements are obtained as the overlap integrals of the initial and final meson wave functions. Their dependence on the square of the transferred momentum (q^{2}) is explicitly determined within the whole accessible kinematic range. A convenient analytical approximation for numerical values of form factors is given. Effects of electroweak physics at short distances and contributions of the intermediate resonances at long distances are taken into account using effective Wilson coefficients. Helicity formalism is employed for the calculation of the differential decay rates and branching fractions of the charm meson into vector light mesons. Reasonable agreement with experimental upper bounds on the rare decay branching fractions is obtained.

The neutrino survival probabilities in experiments with neutrino detection through charged-current and neutral-current interactions have been calculated using a quantum field-theoretical approach with modified Feynman propagators. The method closely resembles the traditional Feynman diagram technique in the momentum representation with both initial and final particle states being described by plane waves, which simplifies the calculations considerably.

Proposals for the creation of the NEUTRONIUM lunar observatory are given. Simulation results and the design of the observatory are presented. An original idea of cosmic ray particle energy measurement for high and ultrahigh energies is proposed. According to this idea, radiation scattered in the opposite direction relative to the direction of arrival of particles (radio waves, neutrons, gamma, charged particles) from hadronic and electromagnetic cascades created by the primary particle in the lunar regolith is registered on the Moon surface.

NA64 is an experiment to search for dark matter particles. Its main goal is to search for the so-called dark photon (A^{prime}) a vector mediator between ordinary and dark matter. One of the possible channels for the production of (A^{prime}) is the invisible decay of the (rho^{0}) meson formed in the photoproduction process. The article presents a simulation of (rho^{0}) meson production and provides an estimate of the number that could be obtained in the NA64 experiment. Possibility of searching for two modes of (rho^{0}) meson decay for different tasks discussed.

For the process of single top quark production within the ‘‘simplified model’’ with a scalar dark matter mediator, a new variable based on angular correlations was presented, for the proper reconstruction of which it is necessary to separate the contributions of two undetectable particles: the neutrino and the mediator. In this work, various machine learning approaches for reconstructing the momenta of these particles are analyzed. A comparison is made between the results obtained using a multilayer perceptron and the Normalizing Flows architectures. The neural networks based on Normalizing Flows, presented in this work, demonstrate a high quality of reconstruction of the target variable and can be used for collider data analysis when applying the unfolding procedure to restore parton-level correlations.

A new approach to spinor field theory is developed on the basis of a non-trivial extension of the Lorentz spin group to a de Sitter group, which leads to a generalized Dirac equation. Within the framework of this model, it is shown that fundamental spin variables generate an induced geometric structure on the spacetime manifold. We clarify the importance of the Kerr–Schild representation in the new field-theoretical approach making it helpful for the study of the chirality issue and a possible violation of Lorentz symmetry.

In this work the theory of diffusive shock acceleration is extended to the case of nonclassical particle transport with Lévy flights and Lévy traps, when the mean square displacement grows nonlinearly with time. In this approach the Green function is not a Gaussian but it exhibits power-law tails. By using the propagator appropriate for nonclassical diffusion, it is found for the first time that energy spectral index of particles accelerated at shock front is (gamma=[alpha(r+5)-6beta]/[alpha(r-1)]), where (0<alpha<2) and (0<beta<1) are the exponents of power-law behavior of Lévy flights and Lévy traps, respectively. We note that this result coincides with standard slope at (alpha=2), (beta=1) (normal diffusion), and also includes those obtained earlier for the subdiffusion ((alpha=2), (beta<1)) and superdiffusion ((alpha<2), (beta=1)) regimes.